Estimate the number of oxygen atoms contained in the amount of oxygen gas that forms when 58.6 g of KNO3 decomposes according to the following chemical equation:

Question

Estimate the number of oxygen atoms contained in the amount of oxygen gas that forms when 58.6 g of KNO3 decomposes according to the following chemical equation:

a) 5.58 × 10^23 atoms

b) 4.36 × 10^23 atoms

c) 0.724 atoms

d) 8.72 × 10^23 atoms

e) 4.36 × 10^-23 atoms

I keep getting b but the answer is D. I'm not sure what I'm doing wrong.

Answer 1

You didn't give the "following equation" for the decomposition of KNO₃. I'm going to assume you meant to include this as the equation:

4KNO₃ ==> 2K₂O + 2N₂ + 5O₂ ... balanced equation

That being the case...

58.6 g KNO₃ x 1 mol / 101.1 g = 0.5796 mols KNO₃

0.5796 mols KNO₃ x 5 mols O₂ / 4 mol KNO₃ = 0.7245 mols O₂

0.7245 mols O₂ x 6.02x10²³ molecules O₂ / mol x 2 atoms of O / molecule O₂ = 8.72x10²³ atoms of oxygen

The reason you keep getting (b) as the answer is because you are calculating molecules of O₂ and not atoms of O. There are 2 O atoms in each O₂ molecule.